One of the biggest differences between a 3-4 and 4-3 is the blitz. With a 3-4, you can send any one of your four LBs and still have the same number of defenders in coverage as a 4-3. This is what the Steelers did to disrupt the Colts offense, because Manning never knew where that blitzing LB was coming from. Of course, it takes very athletic LBs that can shed blocks and penetrate the pocket for a 3-4 to be effective. Linemen are more run stoppers than anything, because they are simply outnumbered at the line of scrimmage against at least five offensive linemen.
When you blitz in a 4-3, you're taking someone out of coverage. But the pressure on the pocket should be consistently coming from your defensive ends, which have to be a little smaller than 3-4 ends, and much quicker to the ball.
Many teams have been switching to 3-4 defenses these days simply because of the talent pool available to them. It is tough to find a good defensive end to make a 4-3 work as designed, so many teams opt for the 3-4 in order to take advantage of the larger pool of available LBs right now.
I've always preferred a 4-3 as a defensive scheme, simply because it seems to offer a more consistent run stopping ability. Having four large dudes on your line as opposed to three seems more conducive to stopping the run.
Some more food for thought:
"The strength of the 3-4 is its ability by its design to confuse the quarterback during passing plays. Most teams generate a pass rush by sending at least 4 defensive men at the quarterback. In a standard 4-3 alignment, these 4 rushers are usually the 4 down linemen. But in a 3-4, the fourth rusher is a linebacker. Since there are 4 linebackers, the fourth potential rusher can come from a variety of spots on the field, thus influencing and sometimes confusing the quarterback's pre-snap defensive read.
A drawback of the 3-4 is that without a fourth lineman to take on the offensive blockers and close the running lane, both the defensive linemen and the linebackers can be overwhelmed by blocking schemes in the running game. 3-4 linebackers must be very athletic and strong enough to shed blocks by fullbacks, tight ends, and offensive linemen to get to the running back.
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The 4-3 is widely used because of its balance in stopping both runs and passes as well as being personnel-friendly since smaller players can be used effectively along the defensive line.
The original version of the 4-3 lined up the tackles over the offensive guards and the end over the offensive tackles, with the middle linebacker over the center and the other linebackers outside the ends. In the mid-1960s Hank Stram developed a popular variation, the "Kansas City Stack", which shifted the strong side defensive end over the tight end, stacked the strongside linebacker over the tackle, and shifted the weakside tackle over center. At about the same time the Cleveland Browns frequently used a weakside shift. The Dallas Cowboys coach Tom Landry developed a "flex" variation, that moved standout lineman Randy White all over. Now every team has its own variations."